Page 17 - Gas Adsorption Equilibria
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b) adsorption isotherms for data correlation allowing to calculate gas-
adsorption-equilibria data at other gas concentrations, pressures and
temperatures.
These data and correlation functions are needed in simulation programs to
develop and check new or better, i. e. smaller, faster and more energy-
efficient adsorption based processes for a large variety of engineering, health
and environmental purposes, cp. Sect. 2.
In Sect. 3 the measurement methods for gas adsorption equilibria which
are presented in this book are outlined. Several other phenomena in gas
adsorption systems like the kinetics of the mass exchange process, which
could not be considered here are mentioned in brief in Section 4. There also
some general information on gas adsorption systems will be given and
references for the various fields mentioned will be provided.
2. GAS ADSORPTION PROCESSES IN SEPARATION
TECHNOLOGY
The sticking of molecules of gases or liquids to the surface of a solid
material is called adsorption. It should not be mixed up with the phenomenon
of absorption where molecules of gases or liquids are dissolved in another
liquid or solid material. Adsorption is a surface phenomenon which in
principle occurs at any pressure and temperature. Absorption is a bulk or
volume phenomenon which may or may not occur at given pressure and
temperature. The difference between both effects simply can be demonstrated
by the sketch shown below. Here the cake symbolizes the molecule of the gas
or liquid. The person represents the solid material. Absorption means eating
the cake. Adsorption occurs if the cake is splashed on the persons face.
The interactions of a gas – normally a mixture – with the surface of a solid
material can be fairly complex. This is due to the fact that the gas molecules
can vary considerably in size, structure and electric properties (dipole and
quadrupole moments), and also the surface of the solid may offer different
types of sites for adsorption, reflected in both the pore spectrum and the
enthalpies of adsorption, cp. Chap. 1, [0.4-0.6]. Hence one has to expect that
interactions between adsorbed molecules of different type will be different
from their possible interactions in a bulk gas or liquid phase.